The present invention relates to method and apparatus for tracking an incrementally sweeping generator by a frequency selective receiver. In order to display the frequency response of a network or transmission line it is customary to arrange for a generator to sweep through the frequency range of interest and either use a broadband receiver to display the output or synchronously tune a selective receiver. Thermal noise or other signals, in the network under test limit the dynamic range of the measurement when a broadband receiver is used. Thus synchronous tuning of a selective receiver is preferred.
The latter may be achieved by careful arrangement of a voltage tuned oscillator being tuned by the same sweep signal that is used to tune the generator. Alternatively the output of one or more reference signals from the generator may be used to ensure that the receiver input is always tuned to the generator output frequency. Of course this process may be reversed in that reference signals from the receiver may be used to ensure that the generator is synchronously tuned. This arrangement is commonly known as a "tracking generator." Examples of arrangements as described above are disclosed in the Hewlett-Packard Application Note 150-3 published August 1972. The general problem of such arrangements is that generator and receiver must be connected by at least one additional signal channel parallel to the network or transmission line under test.
A similar problem arises when it is desired to use a frequency synthesizer as a generator and to use a second frequency synthesizer as the local oscillator in the receiver. When the receiver and generator are physically close then it is possible to devise a common oscillator scheme for synchronous tuning and indeed it is possible to arrange for programming of the two synthesizers even when receiver and generator are separated, by use of a two-way data channel which is used on a "handshake" basis to ensure synchronous sweeping.
Sometimes it is not convenient to provide the separate data channel such as when the distance involved is many miles as in the case of telephone systems when a modem would have to be used and involves the use of serial data so that parallel to serial conversion may become necessary.
One possible solution to this is to arrange for the generator and receiver to step at a precise rate by the use of two identical clock frequencies, so that synchronism is maintained. However, in order to ensure good measurements it is essential to trigger both devices at the same instant and to allow sufficient time at each measurement frequency for both the network under test and the receiver circuits to settle. This normally means that the sweep rate is unnecessarily slow, which is a particular disadvantage of conventional methods of filter testing. Such methods use a constant sweep rate which must be chosen less than or equal to the rate necessary to measure correctly the fastest changing regions of the characteristic of the network under test. This means that the regions of slow change are measured at a rate less than is necessary. Furthermore, the conventional arrangements suffer the disadvantage that if a loss of synchronism occurs, then the system cannot automatically re-lock.